ner kind are used for horn, and still
finer, Pl. 1. Fig. 9. and 10. for metals.

Some of the metals, though not brittle enough to powder under the
pestle, are too soft to be filed, as they clog the file, and prevent its
operation. Zinc is one of these, but it may be powdered when hot in a
heated iron mortar, or it may be rendered brittle, by alloying it with a
small quantity of mercury. One or other of these methods is used by
fire-work makers for producing a blue flame by means of zinc. Metals may
be reduced into grains, by pouring them when melted into water, which
serves very well when they are not wanted in fine powder.

Fruits, potatoes, &c. of a pulpy and fibrous nature may be reduced to
pulp by means of the grater, Pl. 1. Fig. 11.

The choice of the different substances of which these instruments are
made is a matter of importance; brass or copper are unfit for operations
upon substances to be used as food or in pharmacy; and marble or
metallic instruments must not be used for acid substances; hence mortars
of very hard wood, and those of porcelain, granite, or glass, are of
great utility in many operations.

SECT. II.

_Of Sifting and Washing Powdered Substances._

None of the mechanical operations employed for reducing bodies to powder
is capable of producing it of an equal degree of fineness throughout;
the powder obtained by the longest and most accurate trituration being
still an assemblage of particles of various sizes. The coarser of these
are removed, so as only to leave the finer and more homogeneous
particles by means of sieves, Pl. I. Fig. 12. 13. 14. 15. of different
finenesses, adapted to the particular purposes they are intended for;
all the powdered matter which is larger than the intestices of the sieve
remains behind, and is again submitted to the pestle, while the finer
pass through. The sieve Fig. 12. is made of hair-cloth, or of silk
gauze; and the one represented Fig. 13. is of parchment pierced with
round holes of a proper size; this latter is employed in the manufacture
of gun-powder. When very subtile or valuable materials are to be sifted,
which are easily dispersed, or when the finer parts of the powder may be
hurtful, a compound sieve, Fig. 15. is made use of, which consists of
the sieve ABCD, with a lid EF, and receiver GH; these three parts are
represented as joined together for use, Fig. 14.

There is a method of procuring powders of an uniform fineness,
considerably more a